A radiation sensitizer is an agent used to enhance the effect of radiation therapy. In delivering potentially curative doses of radiation, it is necessary to balance the need for local tumor control with the potential for damage to surrounding normal tissues by the delivered dose of radiation (Bush et al., 1978). It is therefore desirable to use the lowest radiation dose consistent with local control. One way to achieve this would be to utilize a radiation sensitizing agent to enhance cytotoxicity of delivered radiation to the tumor.
Radiation causes cell death by damaging critical targets within the cell, most commonly chromosomal DNA (Hendrickson and Withers, 1991). Radiation therapy relies on two types of ionizing radiation: (1) directly ionizing subatomic particle radiation, such as alpha particles and beta particles (electrons), neutrons, protons, mesons, heavy charged ions, etc., and (2) indirectly ionizing electromagnetic radiation, which exists as a family of waves of varying frequency including high frequency x-rays or gamma rays. However, of the two, electromagnetic radiation is more commonly used in radiation therapy today. In tissue, electromagnetic radiation in the form of x-rays or gamma rays can interact with molecules (especially water) causing the ejection of high-energy electrons. The electrons can break the sugar phosphate bonds in DNA directly (direct action) or the process of electron ejection can ultimately produce free (uncharged) radicals that can also break the chemical (sugar-phosphate) bonds in DNA (indirect action). The damage caused through the indirect mechanism is more significant (Hendrickson and Withers, 1991; Mulcahy et al., 1993; Rubin and Siemann, 1993; Chapman et al., 1974). These damaging effects are mediated by the radiation products of water as shown: ##STR1##
Radiation damage is produced primarily by, the hydroxyl radical, HO.sup.574, an oxidizing radical. This radical is extremely reactive and short lived. It causes damage primarily in the vicinity in which it is generated (.+-.4 nm). If it comes into contact with a hydrated electron (e.sup.-.sub.aq), it is deactivated by conversion to a hydroxide ion (OH.sup.-). Hydrated electrons are strong reducing species and highly energetic. They are very mobile by comparison to the hydroxyl radical, can travel distances quickly, and through direct action can damage DNA. However, as mentioned above, they also deactivate hydroxyl radicals readily. Agents with strong electron affinity, by virtue of "soaking up" solvated electrons, prevent them from neutralizing hydroxyl radicals and thereby allow hydroxyl radicals to exert their effect (Adams and Dewey, 1963). Oxygen and other compounds with strong electron affinity would thus be expected to act as radiation sensitizers.
The biological responses to radiation-induced cell injury may be modulated by various endogenous and exogenous compounds, and failure of radiation therapy to achieve local cure is multifactorial. For instance, sulfhydryl compounds, including cysteine, dithiothreitol, and cysteamine have been shown to protect living cells against the lethal effects of ionizing radiation by acting as reducing agents (Rubin and Siemann, 1993) and facilitating the recombination of the ion pairs. It also has been observed that depletion of cellular sulfhydryl compounds can result in radiosensitization.
One of the major factors mediating failure of radiation therapy, or radioresistance, is hypoxia. Hypoxic cells in solid tumors have been observed to be 2.5-3 times more resistant to the damaging effect of ionizing radiation (Tannock, 1972; Watson et al., 1978; both cited in Brown, 1984). Local cure/control rates of a tumor can be increased with an effective increase in the radiation dose; however, such an increase would damage adjacent, fully-oxygenated normal tissues to a greater degree than the tumor cells (Shenoy and Singh, 1992). Specific modification of tumor radiosensitivity has been pursued through alteration of the tumor oxygenation state achieved by fractionation of the radiation dose and by the attempted use of chemical radiation sensitizers (Wang, 1988; Shenoy and Singh, 1992).
Fractionation results in reduced radiation effects in normal tissue as compared with a single acute dose due to cell repopulation and repair of sublethal damage between dose fractions. In malignant tumor tissues, radiosensitive oxygenated cells are destroyed with a subsequent reduction in tumor size. Subsequently, radioresistant hypoxic cells distant from functional vasculature become reoxygenated and therefore more radiosensitive. Reassortment of cells within the cell cycle also occurs and renders the cancer cells more radiosensitive. This differential response between tumor and normal cells may allow dose fractionation to be more tumoricidal than an equal single radiation dose.
Various types of electron-affinic reagents are known to promote radiosensitization of , cells with diminished oxygen supply (Shenoy and Singh, 1992). However, few of these show activity at non-toxic doses in vivo. For instance, clinical trials with one of the better known agents, misonidazole, demonstrated that it is highly effective against a number of animal and human tumors (Thomlinson et al., 1976; Ash et al., 1979; Denekamp et al., 1980; all cited in Brown, 1984). However, the neurological side-effects severely limit its clinical usefulness (Kallman, 1972; Dische et al., 1977; Urtasun et al., 1978; Waserman et al., 1979; all cited in Brown, 1984; and Dische et al., 1979). Approaches aimed at improving the therapeutic index of nitroimidazoles have included lowering the lipophilicity so as to restrict nervous tissue penetration and toxicity, and accelerating renal clearance (Beard et al., 1993). Clinical trials with these second-generation analogs of misonidazole have been reported or are on-going (Roberts et al., 1984; Coleman et al., 1984; Saunders et al., 1984; Coleman et al., 1986; Horwich et al., 1986; Newman et al., 1986; Dische et al., 1986; Coleman et al., 1987; Newman et al., 1988; Workman et al., 1989). However, the approaches have yet to produce highly effective hypoxic cell sensitizers.
Halogenated pyrimidines also have been studied as radiation sensitizers. These agents modify the radiosensitivity of cells through structural alteration of the DNA, making the DNA more susceptible to radiation inactivation. However, the drugs must be present in the cells for extended periods since the degree of radiosensitization is directly related to the degree of thymidine substitution. In addition, the agents may undergo rapid hepatic degradation and dehalogenation (Shenoy and Singh, 1992). The main limiting factor from prolonged use of halogenated pyrimidines has become bone marrow toxicity (Kinsella et al., 1984a; Kinsella et al., 1984b; Kinsella et al., 1985; cited in Shenoy and Singh, 1992).
Hypoxic cell sensitizers fall within the broad category of chemical modifiers of cancer treatment. Chemical modifiers are usually not cytotoxic by themselves but modify or enhance the tissue response to standard radiation therapy. The ultimate utility of a radiotherapy or chemotherapy modifier depends upon its ability to alter the therapeutic index.
Texaphyrins have been described in U.S. Pat. Nos. 4,935,498, 5,162,509, 5,252,720, 5,292,414, 5,272,142, 5,457,183 and 5,256,399; U.S. applications Ser. Nos. 08/196,964, 08/294,344, and 08/227,370; and PCT/US94/06284, all of which are incorporated by reference herein. The photophysical properties of various texaphyrins are reported in U.S. Pat. No. 5,252,720, incorporated by reference herein, and include strong low energy optical absorptions in the 690-880 nm spectral range, a high triplet quantum yield and efficient production of singlet oxygen. U.S. Pat. No. 5,252,720 also describes photosensitized inactivation of enveloped viruses and magnetic resonance imaging (MRI) of atheroma, liver, kidney and tumor using various substituted texaphyrin metal complexes. Altering the polarity and electrical charges of side groups of these macrocycles alters the degree, rate, and site(s) of binding to free enveloped viruses such as HIV-1 and to virally-infected peripheral mononuclear cells, thus modulating photosensitizer take-up and photosensitization of leukemia or lymphoma cells contaminating bone-marrow. Powerful techniques include the use of these texaphyrins in magnetic resonance imaging followed by photodynamic tumor therapy in the treatment of atheroma, and benign and malignant tumors.
The present invention provides texaphyrins for radiation sensitization. Texaphyrins enhance radiation damage and overcome many of the drawbacks of prior art radiation sensitizers.